WASHINGTON  Roland W. Burris, the would-be junior senator from Illinois, picked up the support of a key Democrat late Tuesday afternoon in his bid to occupy the seat vacated by President-elect Barack Obama.

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Mr. Burris, who was rebuffed by the Senate clerk earlier in the day, gained the support of Senator Dianne Feinstein of California, the chairman of the Rules Committee, who broke with many of her Democratic colleagues and said that Mr. Burris should be seated despite having been appointed by Gov. Rod R. Blagojevich, who is facing corruption charges.

Regardless of the charges against him, the governor still has the right to fill the Senate vacancy, Ms. Feinstein said, and keeping Mr. Burris from taking his seat could have implications for appointments by other governors.

Ms. Feinstein’s support is important because her committee has a say in whether Mr. Burris is qualified to serve. The California senator’s endorsement was surely heartening for Mr. Burris, who arrived at the Capitol on Tuesday morning for the start of the 111th Congress to meet a mob of reporters and photographers. Mr. Burris, who shortly before his arrival had insisted he was “certainly not looking for drama,” found himself caught up in a comedy of sorts. Looking unsettled and with rain glistening on his topcoat, he entered the building to encounter a mob of reporters and photographers in a spectacle that briefly overshadowed the convening of the new House and Senate that will soon take up the ambitious program of Mr. Obama.

Capitol police officers tried to clear a path for Mr. Burris. “You can’t keep a regular citizen from walking into the Capitol,” one officer shouted.

Somehow, Citizen Burris made his way to the office of Nancy Erickson, the secretary of the Senate, to whom he presented his credentials, only to have her reject them. Afterward, the aspiring legislator stood in the rain outside and declared, “Members of the media, my name is Roland Burris, the junior senator from the State of Illinois.”

Not yet, he isn’t, although it remained to be seen whether Senator Feinstein’s endorsement would sway other senators. The problem for Mr. Burris, of course, is that he was named to the seat by Mr. Blagojevich. Ms. Erickson had already said that the appointment letter forwarded by the governor’s office did not comply with Rule II of the Senate’s standing rules, which requires signatures of both the governor and the secretary of state.

The Illinois secretary of state, Jesse White, has refused to sign, saying the appointment is invalid because of a federal corruption investigation surrounding the governor and what prosecutors describe as his efforts to sell the Senate seat, vacated by Mr. Obama.

“Roland Burris is welcome to Washington; he is welcome to come to my office,” Illinois’s senior senator, Richard J. Durbin, the chamber’s No. 2 Democrat, said on Monday. “But he couldn’t possibly be anticipating any official action until at least his papers are filed.”

For a short time, the swirl around Mr. Burris eclipsed the main event at the Capitol: the swearing-in of the new Congress, in which Democrats will have more power and will soon wrestle with the new president’s economic stimulus program.

“We will hit the ground running,” House Speaker Nancy Pelosi of California promised.

After the mob scene at the Capitol on Tuesday morning, Mr. Burris and his lawyers held a brief news conference. One lawyer, Timothy W. Wright III, complained that it was “against the law of the land” to block Mr. Burris from taking his seat.

Mr. Wright vowed to do whatever necessary to “resolve this impasse and move forward on behalf of the people.” The lawyer said Mr. Burris would continue to confer with the Senate leadership and might go to court if the talks are unsuccessful.

If the Illinois Senate seat is contested in court, the issues could be serious indeed. The Constitution gives each chamber of Congress the power to judge the qualifications of its members. On the other hand, the Supreme Court has ruled that the lawmakers cannot arbitrarily add membership requirements that are not spelled out in the Constitution.

And if the appointment of Mr. Burris is tainted by Mr. Blagojevich’s situation, as some have argued, it must be noted that the governor has only been accused of crimes, but neither indicted nor convicted.

(After Mr. Burris was turned away, Mr. Blagojevich issued a statement in Chicago in which he called Mr. Burris “a good and decent man with a long history of public service” and declaring that “any allegations against me should not be held against him and especially not the people of Illinois.”)

The uncertainty over the Illinois seat highlighted the first day of the 111th Congress, a day likely to be marked by more ceremony than substance, with new members of the House and Senate being sworn in and learning their way around the Capitol corridors.

Senate Democrats were also considering when they might make an effort to seat Al Franken of Minnesota, the Democrat who was declared the winner Monday of a prolonged recount of votes, though he cannot be certified by the state as senator for at least a week.